This invention concerns the manufacture of iron oxide. More particularly, it concerns a method of producing a highly acicular, crystalline, yellow iron oxide. The utility of the invention resides in the production of pigmentary iron oxide.
The general technology of producing pigmentary iron oxide is well-known and is well summarized in the disclosure of U.S. Pat. No. 2,939,767. Broadly, the procedure involves oxidizing ferrous ion to ferric ion under conditions such that FeOOH, sometimes written as Fe.sub.2 O.sub.3.H.sub.2 O or Fe.sub.2 O.sub.3.xH.sub.2 O, is obtained as the yellow oxide of iron (also called alpha ferric oxide or ferric oxide hydrate). This procedure usually takes place when the pH of the reaction is below 7, such as from 2.5 to about 4.0, so that the reaction environment is considered "acid". It is also possible to form hydrated ion oxide in an alkaline environment, as disclosed in U.S. Pat. No. 2,558,302. Both of these procedures result in many operating difficulties, leading to varying efficiencies of product yield and variable pigment colors.